var mergeTwoLists = function(list1, list2) {
    let l1 = list1
    let l2 = list2
    let res = new ListNode()
    let head = res

    while (l1 && l2) {
        if (l1.val <= l2.val) {
            res.next = l1
            l1 = l1.next
    
        }else{
            res.next = l2
            l2 = l2.next
    
        }
        res = res.next

    }
    res.next = l1 !== null ? l1 : l2

    return head.next

};